In recent years, various plastic materials have been used in, for example, electronic parts.
In these fields with the need for particularly high heat resistant reliability, heat resistant films such as polyimide films and polyether imide films are mainly employed and heat-curing adhesives are employed in adhesion of these films. In the fields where the necessity for high heat resistant reliability is not so strict, there have been employed polyester films and pressure-sensitive adhesives for adhesion of the same.
The pressure-sensitive adhesive provided with a release paper is bonded as such to electronic parts or the like and then subjected to a mounting step such as solder reflow, and after the release paper is peeled, the pressure-sensitive adhesive is adhered to a polyester film. In such a case, it is required that these pressure-sensitive adhesives have a high heat resistance almost comparable to the level of heat-curing adhesives, as well as strong adhesion to a polyester film.
There are various pressure-sensitive adhesives showing tackiness at ordinary temperature. Also, various pressure-sensitive acrylic adhesives are known as being usable in adhesion of electronic parts. Pressure-sensitive acrylic adhesives usually contain, as the main component, a tacky copolymer of a monomer mixture which is composed of an alkyl (meth)acrylate carrying an alkyl group having 2 to 14 carbon atoms on average as the main component together with a monoethylenic unsaturated monomer such as acrylic acid.
Although these pressure-sensitive acrylic adhesives show heat resistance of a certain extent, they are poor in the adhesive strength to polyester films and thus sometimes cause troubles such as peeling. When tackifier resins commonly employed in the art are added thereto to improve the adhesive strength to polyester films, these adhesives are softened and fluidized in the step of solder reflow, which makes them unusable.